The present invention relates to an EDM apparatus wherein the electrode workpiece is cut by electrical discharges by means of an electrode tool in the form of a longitudinally traveling wire, and more particularly to a traveling wire EDM apparatus provided with means for threading the free end of the electrode wire through a hole pre-drilled in the workpiece, and for capturing the end of the wire passed through the pre-drilled hole by the feeding mechanism feeding the wire longitudinally through the machining zone.
Such an arrangement, as provided by the present invention, for automatically threading a wire electrode does away with one of the most difficult manual operations required in traveling wire EDM apparatus prior to effecting a cut through the workpiece. In view of the very small diameter of the wire and of the thermic and mechanical stresses exerted on the wire during a cutting operation, rupture of the wire may occur at any point of the cutting path. However, when the wire ruptures, the slit cut in the workpiece is too narrow to permit automatic rethread of the wire electrode at the point of rupture by means of the automatic rethreading mechanism used for effecting the original threading of the electrode wire through the pre-drilled hole, and the machine operator must intervene to manually effect the rethreading operation. Consequently, the EDM apparatus must be continuously monitored by the machine operator.